The prior frame has a first side C-shaped groove member, a second side C-shaped groove member and an intermediate C-shaped groove member. The first side C-shaped groove member and the second side C-shaped groove member are linked to each other with an angle through the intermediate C-shaped groove member. In other words, a middle section of the first side C-shaped groove member is connected to a most left section of the intermediate C-shaped groove member, a right section of the first side C-shaped groove member is connected to an upper left section of the intermediate C-shaped groove member, a middle section of the second side C-shaped groove member is connected to a most right section of the intermediate C-shaped groove member, and a left section of the second side C-shaped groove member is connected to an upper right section of the intermediate C-shaped groove member, thereby forming a trapezoidal structure with four connection points. The first side C-shaped groove member, the second side C-shaped groove member and the intermediate C-shaped groove member have grooves all towards the same direction. In the prior frame, the first side C-shaped groove member and the second side C-shaped groove member are linked to each other through the intermediate C-shaped groove member. The first side C-shaped groove member and the second side C-shaped groove member are linked to each other with a gap instead of direct connection. Since the first side C-shaped groove member and the second side C-shaped groove member are not directly connected to each other, it readily results in the production of stress and strain in the gap, and thus the overall structure of the frame is not strong enough. Therefore, it is necessary to provide solutions to improve the prior frame.